The present invention relates generally to improvements in a focussing plate and it relates more particularly to an improved focussing plate on one surface of which there are orderly distributed and formed innumerable minute conical projections or cavities of a conical shape.
A prior art focussing plate has on one face a light diffusing surface on which a large number of minute irregularities are randomly formed. However, such a prior art focussing plate poses a problem that light is diffused in all directions so that only a small amount of light reaches the observer's eye, thereby presenting a dark image, when viewed. Attempts to avoid this shortcoming have been proposed in U.S. Pat. No. 2,589,014 and Japanese laid open patent application SHO No. 48-95827 which provide a focussing plate having on its surface minute conical projections, pyramids or cavities of a conical shape which are orderly arranged, rather than minute irregularities. FIGS. 1 (a), (b), (c) and (d) of the drawings herein represent conical projections formed on one surface of a focussing plate proposed in the above publications. With the focussing plates thus proposed, emergent light tends to be directed in specific directions commensurate with the angle of slope of the conical projections or cavities, so that a suitable selection of an angle of slope leads to the presentation of an image having almost the same brightness as that of an image obtained through a transparent plate.
However, such focussing plates having the aforesaid conical projections or cavities suffer from another shortcoming in that the appearance of a defocussed image is entirely different from the appearance of an image formed on the surface of a film. FIG. 2 shows the optical paths of light rays or fluxes from a light source (a photographic object) which have passed through the focussing plate of a type shown in FIG. 1(a). The light rays or fluxes I, II, III passing through an objective lens from a light source are divided into light fluxes A and B; C and D; and E and F, respectively, due to the refraction through the surfaces of the respective conical projections, being emergent at angles inclined to the incident angles of the light rays through the sloped surfaces of the respective conical projections. However, when the observer's eye is positioned on an optical axis, then only a light flux E', which is part of the emergent light flux E of the light flux I that has passed through an intermediate annular zone of an exit pupil of an objective lens reaches the observer's eye with the result that a defocussed image of a light source may be observed as a circle of confusion of ring shape, as shown in FIG. 3. The appearance of a defocussed image as shown in FIG. 3 is entirely different from that of an image on the surface of a film, the latter appearance giving a uniform density of image over the entire surface of the film resulting in the failure to obtain a picture by estimating the appearance of the image of the film from the appearance of the image on the focussing plate. With the aforesaid focussing plate, light fluxes which enter the observer's eye, as shown in FIG. 3, are only light fluxes which have passed through the intermediate annular zone or its outer peripheral portions in the exit pupil of the objective lens, so that when an objective lens is stopped down, then the above fluxes are blocked by a diaphragm, resulting in shading in the field of view. In addition, when used in combination with a Fresnel lens, then there appears moire patterns in the field of view, thus hindering satisfactory observation. Furthermore, when a light receiving element for use in TTL (through the lens) light measurement is placed in the rear of the focussing plate, then there results over-exposure or under-exposure depending on whether the light receiving element is positioned within a circle of confusion as shown in FIG. 3.